This invention relates to a misfiring sensing apparatus for an internal combustion engine, and in particular to a misfiring sensing apparatus for an engine of an automobile.
Misfiring is a phenomenon which occurs in an internal combustion engine when one of the cylinders in the engine fails to ignite. Misfiring can be due to a number of causes, such as the failure of the ignition system to generate an adequate spark in a cylinder or a failure of the fuel supply system to supply a proper fuel-air mixture to a cylinder. When misfiring is due to a failure of the ignition system, uncombusted fuel is discharged from a misfiring cylinder. The discharge of uncombusted fuel from an engine is undesirable, because the uncombusted fuel can damage a catalytic converter for the engine. Therefore, it is desirable to be able to detect misfiring in an engine and then cut off the supply of fuel to a misfiring cylinder.
For this reason, various misfiring sensing devices have been proposed. One conventional type of misfiring sensing device is disclosed in Japanese Published Unexamined Patent Application No. 62-26345. In that device, the pressure within each cylinder of an engine is sensed by a pressure sensor, and the crankshaft angle at which a maximum pressure occurs in each cylinder is determined. The engine is considered to be operating normally when the maximum pressure for each cylinder falls within a prescribed period defined by two crankshaft angles.
However, that conventional device has a number of problems. For example, it is necessary to continuously monitor the cylinder pressure, so the apparatus tends to be complicated. Furthermore, when the engine is operating under a light load, the pressure within each cylinder has two peaks, one occurring at top dead center during the compression stroke and the other occurring during the combustion stroke, and it is difficult to differentiate the two peaks. Furthermore, when a pressure peak occurs prior to top dead center during the compression stroke of a cylinder, it is not possible to detect misfiring.